This invention relates to a communication control system, a radio communication system, a mobile station, a base station and a base station control unit, and particularly to a packet transfer system utilizing re-transmission control.
In a WCDMA cellular system, a base station can use a plurality of different frequency bands, and a mobile station is capable of using a hand-over between different frequencies. Also, sometimes, hand-over to a different type of cellular system such as a GSM may be made. For this reason, each mobile station requires time for reception quality measurement, synchronization establishment, etc. of the hand-over destination frequency and by frequency switching. To permit this, a compressed mode (CM) is set in a WCDMA. In the CM, transmission and reception of some transmitted frames via a link being in communication are interrupted to provide a transmission gap permitting the mobile station to measure the different frequency band. The data during the transmission gap is transmitted as compressed data at an increased transfer rate before and after the interruption. The base station control unit sets up the following parameters as shown in FIG. 8, and informs the setup parameters to the base and mobile stations. The base and mobile stations are set in the CM according to these parameters:
Transmission Gap (TG)
Transmission Gap Length (TGL)
Transmission Gap Distance (TGD)
Transmission Gap Pattern Length (TGPL) and
Transmission Gap Pattern Repetition Counter (TGPRC)
The TG can be set for the uplink only, the downlink only or both the uplink and downlink.
In packet transfer in the WCDMA system, error detection is made in a radio unit controller in the base station control unit. The operation of error detection is executed in one of two modes, i.e., an acknowledged mode (AM) and an unacknowledged mode (UM). In the AM mode, when the radio link controller detects any error, it transmits a control signal for requesting re-transmission to the mobile station. In the UM, when the radio link controller detects any error, it discards the pertinent data block, and does not request any re-transmission. The setting of either the AM or the UM can be made for each data flow. The AM is set in case of application with a very low request error factor, for instance, a file transfer. The UM is set in case of an application that requires a real time property, for instance, a VoIP.
Usually, the error factor in the RLC must be controlled to be very small because, in case of the AM in which re-transmission is made, although a block error can be avoided, the connection of a plurality of base stations to the base station control unit leads to very long control delay. Therefore, re-transmission delay is long, resulting in a very long packet transmission delay. For example, in case of the file transfer, the file transfer time is increased. In case of the UM, since any error detected data is discarded, data loss is increased at the application level. For example, in case of the VoIP, the voice quality is greatly deteriorated.
Although the error factor in the radio layer can be reduced by increasing the transmission power per bit, a large power is required in order to always maintain a low error rate because the radio propagation environment tends to largely change due to fading or the like. This leads to increase of interference components and consumption of power resources, thus reducing the system capacity. Accordingly, in case of an HSDPA (High Speed Downlink Packet Access) or an EUDCH (Enhanced Uplink Dedicated Channel), which is a high rate transfer system in the WCDMA, the base station executes an HARQ (Hybrid Automatic Re-transmission Control) to largely reduce the error factor in the radio layer. The HARQ is a method in which a check is made to determine if there is any data block reception error. If there is any reception error, the same data block is re-transmitted. On the receiving side, the first transmitted and re-transmitted data blocks are combined to increase the probability of obtaining a correct data block. This method will be described in greater details by taking the EUDCH, i.e., Uplink High Speed Packet Transfer, as an example. The base station checks if a data block is received without any error. If the base station detects any error, it transmits a NACK signal according to a downlink control signal. If the base station does not detect any error, it transmits an ACK signal. If the mobile station receives the NACK signal, it re-transmits the same data block at the predetermined re-transmission timing. If the mobile station receives the ACK signal, it transmits a new data block at the predetermined transmission timing. If the base station transmits the NACK signal, it holds the error detection data block in a buffer, and combines the data re-transmitted from the mobile station with the held data. The base and mobile stations execute data block re-transmission until data block is correctly received or the re-transmission reaches the predetermined maximum number.
With the HARQ introduced into the base station, it is possible to effect the re-transmission at a higher rate than in the case of re-transmission in the RLC, thus reducing the data transmission delay in the AM and improving the data reaching factor of the UM data.
However, as described hereinbefore, in the WCDMA the TG may sometimes be set in the CM. FIG. 9 shows a case, in which a mobile station, while in the EUDCH, starts to be set in the CM. In the EUDCH, the time interval T_BS from the instant when the base station receives a packet to the instant when the base station transmits an error detection result (ACK/NACK signal), is fixed. Therefore, concerning a packet having been received time T_BS before the TG, the base station cannot transmit the ACK/NACK signal. If the mobile station could not receive the ACK/NACK signal after the predetermined time, it decides that the base station could not detect any packet signal. Actually, even if, for instance, the base station could not receive correctly, the mobile station always executes packet re-transmission. Therefore, the wasteful packet transmission poses such problems as reduction of uplink use efficiency and increase of packet transmission delay.
Accordingly, in the present HSDPA it is prescribed that in case when the ACK/NACK signal transmission timing overlaps the TG, the base station does not transmit any ACK/NACK signal. The mobile station is thus scheduled that it does not transmit any packet at such timing (Literature 1: 3 GPPTS25.214V6.3.0 (2004-09) 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Physical Layer Network (FDD)).
Literature 2 (3 GPPRAN WG1 38bis Meeting R1-041178 “E-DCH and Compressed Mode” (Ericsson)) proposes that in the EUDCH, the mobile station transmits packets even at such timings that the ACK/NACK signal frames overlap the TG, and that the base station transmits only some of the ACK/NACK signals not overlapping the TG. As shown in FIG. 10(a), the radio channel is arranged such that one transmission time interval (TTI) is divided into five sub-frames and that the base station is adapted to repeatedly transmit five ACK/NACK signals in sub-frame units. In the TTI having overlap over the TG, the base station transmits ACK/NACK signals only in sub-frames free from over lap over the TG. Thus, packet transmission is realized regardless of presence or absence of the TG. Literature 3 (3 GPPRAN WG1 Release 6 Ad Hoc Meeting R1-040770 “Interaction between Enhanced Uplink and Compressed Mode” (Philips)) proposes, as shown in FIG. 10(b), that in case of the ACK/NACK signal overlapping over the TG in the EUDCH, the base station transmits the ACK/NACK signal in the next TTI. Like the case of Literature 2, packet transmission is realized regardless of presence or absence of the TG.
The above prior art techniques, however, have the following problems. In the method according to the Literature 1, despite the packet transmission, a time interval incapable of transmitting packets arises at the time of the ACK/NACK signal overlapping over the TG. This poses a problem of thorough-put reduction. Also, in case of such data flow as the VoIP which uses the UM and attaches importance to the real time property, there is a time interval when a data block can not be transmitted as soon as it is generated. Therefore, such problems as increase of delay time swaying and service quality deterioration are posed. In this method, it is possible to use a transmission time interval of 10 ms in addition to 2 ms. Accordingly, when setting as in the case of the HSDPA is applied to the EUDCH, the time interval incapable of transmission is increased to increase the adverse effect of service quality deterioration. In case of the transmission time interval of 2 ms, since the TG can be set to about 10 ms at the most, a case may arise that no packet can be transmitted in about 10 ms.
According to the Literatures 2 and 3, it is possible to solve the problem that a data block cannot be transmitted as soon as it is generated as in the Literature 1. However, the following problems are still posed. According to the Literature 2, since only some of the ACK/NACK signals by holding the transmission power of the base station constant, the ANK/NACK error factor is increased. The increased ACK/NACK signal error factor increases wasteful re-transmission packets (error of ACK→NACK). Therefore, the transmission of new packets is interfered with, and the thorough-put is deteriorated. Also, in the HARQ, re-transmission fails to execute correctly (error of NACK→ACK). Therefore, re-transmission occurs in the RLC, and the packet transmission delay is extremely increased. Furthermore, in order to maintain the ACK/NACK signal error factor to be substantially the same as in the case without overlap over the TG, the transmission power of the base station must be increased.
According to the Literature 3, since the ACK/NACK signals in the case of overlapping over the TG are transmitted in the next TTI, there causes a problem to increase the packet transmission cycle. In order to solve this problem, it is necessary to change the timing of the ACK/NACK signal transmission and reception in the base and mobile stations depending on the result of detection of overlapping over the TG is detected. Therefore, the operation in each station is complicated. Also, since the ACK/NACK signal is shifted to the next TTI, it is impossible to transmit the ACK/NACK signal concerning the packet, in which the ACK/NACK signal is to be transmitted in the pertinent TTI. Consequently, a time interval incapable of transmitting packet arises, thus posing the same problems as in case of the Literature 1, i.e., increase of delay swaying and deterioration of service quality.